Incandescent lamp having linear output

ABSTRACT

The coiled filament of an elongated tubular incandescent lamp is axially mounted and supported therein by spacers bearing against the inside wall of the lamp envelope. The space between filament turns is less at its ends than at its center in order to yield a substantially uniform output of radiational energy from the entire length of the lamp.

United States Patent Inventor Appl. No. Filed Patented AssigneeINCANDESCENT LAMP HAVING LINEAR OUTPUT 5 Claims, 1 Drawing Fig.

U.S.Cl 313/274, 313/315 Int.Cl I-I01j1/96,

' 1-101j 19/52 Field of Search 313/271, 274,315

Primary ExaminerDavid Schonberg Assistant Examiner-Paul A. SacherAttorneys-Norman J. OMalley and James Theodosopoulos v ABSTRACT: Thecoiled filament of an elongated tubular incandescent lamp is axiallymounted and supported therein by spacers bearing against the inside wallof the lamp envelope. The space between filament turns is less at itsends than at its center in order to yield a substantially uniform outputof radiational energy from the entire length of the lamp.

Patented May 18, 1971 3,579,021

STEPHEN F. KIMBALL INVENTOR AGENT 1 INCANDESCENT LAMP HAVING LINEAROUTPUT BACKGROUND OF THE INVENTION 1. Field of the Invention Thisinvention relates to the field of elongated tubular incandescent lampsand particularly to such lamps having a sufficiently high output as torequire a high temperature envelope, such as high silica glass.

2. Description of the Prior Art In the past, elongatedtubular-incandescent lamps of the type relating to the field of thisinvention have generally had a filament which was substantiallyuniformly coiled throughout its length. Such lamps have been made inlengths of up to 2 or 3 feet and have generally been used for heating. Aprofile of the radiational output of such lamps taken along the lengthof the lamps but a short distance therefrom is greater at the centerthan at the ends. In fact, for a lamp having uniform coiling throughoutits length, the output at the center of the lamp is about double that atthe ends thereof, when the output is measured on a line parallel to thelamp but only a short distance therefrom. This disparity in the outputprofile is increased even more by the fact that the ends of the lampfilament operate at a lower temperature, due to end cooling, and emitless radiational energy than does the center.

In many commercial applications where such lamps are used as infraredheaters, the nonlinear output has little practical effect since theheating requirements are not critical. In addition, the lamps in thoseapplications are generally spaced far enough from the materials beingheated so as to reduce or substantially eliminate the variation in theoutput profile.

It is to be remembered that the profile variation is greatest whenmeasured at a distance from the lamp that is relatively short inrelation to the lamp length.

Recently, photocopying applications have arisen where such lamps areused at a relatively short distance from the material being heated orilluminated. A typical application is in reprographic or photocopyingequipment where an illuminating lamp can be positioned a short distancefrom a master copy being reproduced. Or a heating lamp can be used tofuse a thermoplastic film on the reproduction. In these applications,the photo copying is conducted at such high speeds and at such proximityto the lamps, that the output profile thereof is critical. A nonuniformoutput as is obtained from a lamp having a filament that has uniformturn spacing throughout is generally unsatisfactory for suchapplications.

In some cases, also, there are sufficient edge losses in thephotocopying equipment as to require a lamp having higher output at itsends than at its center in order to compensate for the losses.

SUMMARY OF THE INVENTION We have found that the variation in outputprofile of elongated, tubular, high temperature lamps can be markedlyreduced when the ends of the filament thereof are caused to operate athigher temperatures than the center of the filament. More specifically,we coil the ends of the filament at a higher pitch, that is, at a highnumber of turns per inch (TPI), than the center of the filament. Thecloser proximity to each other of the turns at the end results in ahigher operating temperature thereat than at the center. The radiationalenergy emitted therefrom is thus greater than at the center, the effectof which is to be substantially improve the linearity of the outputprofile over that of the prior art lamps.

The improvement in linearity is directly dependent on, among otherthings, the ratio of the turn spacing at the ends of the filament tothat at the center. A ratio only slightly greater than 1.0 will improvethe linearity only slightly. However, there may be situations where sucha ratio, say, 1.05 or 1.10, is satisfactory, as where the ends of thefilament extend beyond the edges of the paper being heated orilluminated and where the distance between the lamp and the paper isless than about one-twentieth of the lamp length. In such a situation,the nonlinear output of a lamp has less effect than in one where thefilament extends only to the edges of the paper and where the distancebetween the lamp and the paper is about one-fifth or one-tenth of thelamp length. In this case, a ratio greater than 1.1 would preferably benecessary to obtain a satisfactory amount of radiational energy on theentire surface of the paper being heated or illuminated. However, theturn spacing at the ends of the filament should be great enough toprevent any substantial shorting out of individual turns.

In addition, the increased pitch at the filament ends must not extendexcessively along the length of the filament; otherwise, the output ofthe higher pitch end itself would be excessively nonlinear. Generally,the higher pitched coiling should not extend more than one-third of thecoiled filament length from the end thereof.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE is an elevationalview of a lamp in accordance with this invention showing a filamenthaving higher pitch turns at its ends than at its center.

DESCRIPTION OF THE PREFERRED EMBODIMENT Filament 3 has integral terminalportions 4 at each end, the

pitch of terminal portions 4 being greater than the pitch of themainbody of filament 3. As previously mentioned, each terminal portion 4should be less than one-third the total length of filament 3. Filament3, including terminal portions 4, preferably consists of a single lengthof coiled wire, preferably tungsten, in order to avoid mechanical andelectrical connections, for example, welds, between the ends of filament3. Lamps of the type under consideration herein generally operate atsuch a high temperature that such connections could be locations ofearly failure of a filament. Of course, filament 3 can consist of 2 ormore wires wound together in parallel on the same mandrel for thepurpose of increasing the current carrying capacity of a filamentwithout increase in the diameter of the filament wire. Filament 3 issupported throughout its length by metallic spacers 8 bearing againstthe inner wall of envelope 2.

The ends of filament 3 are connected to ribbon connectors 5 embedded inpress seals 6 at each end of envelope 2. Ceramic bases 7 are attached tothe ends of press seals 6 and ribbon.

connectors 5 are electrically connected to external contact terminals inbases 7.

In particular example of a 1,000-watt, T2%, -volt quartz-halogeninfrared lamp in accordance with this invention, envelope 2 had a lengthof 17% inches and a diameter of 8 millimeters. Filament 3 consisted oftwo 3,660 millimeter lengths of 9.95 mil tungsten wire bifilarly woundon a 35 mil mandrel. The mandrel, of course, was removed prior to themounting of filament 3 in envelope 2. Filament 3 had an overall lengthof 15% inches and was supported throughout the length of envelope 2 bywound tungsten spacers 8 spaced about one-half inch apart. The ends offilament 3 were connected to molybdenum ribbons 5 by lead-in wires 9.One end of lead-in wire 9 was welded to ribbon 5 and the other end 10was coiled, thereby permitting the end of filament 3 to be threadedthereinto to provide a secure mechanical and electrical connection.

Terminal portions 4 of filament 3 were each 3 inches long which wasabout 20 percent of the total length of filament 3. Terminal portions 4were coiled at 43 turns per inch while the central portion of filament 3was coiled at 35 turns per inch, the ratio therebetween being 1.22 to 1.At normal operation the filament temperature at terminal portions 4 was2,343 K. and the filament temperature at the center was 2,245 K. Thusterminal portions 4 operated at a temperature 98 K. higher than thecenter of filament 3 and the increased radiation therefrom resulted in alamp output profile that, when measured at a distance of 1 inch from thelamp, was more linear than a control lamp, the filament of which haduniform turn spacing throughout its length.

Iclaim:

1. An incandescent lamp having improved uniformity of radiational outputalong the length of the lamp comprising: an elongated tubular highsilica glass envelope having press seals at each end thereof; acontinuously coiled tungsten filament axially disposed within saidenvelope, said filament having coiled integral terminal portions, saidcoiled portions having a greater pitch than the central portion of saidfilament; and

spacers supporting said filament within said envelope.

2. The lamp of claim 1 wherein the length of each of said terminalportions is less than one-third the total coiled length of saidfilament.

3. The lamp of claim 1 ,wherein the space between turns of said terminalportions is sufficient to prevent any substantial shorting out ofindividual turns.

4. The lamp of claim 1 wherein the ends of said filament are threadedinto coiled lead-in wires, said lead-in wires being connected to ribbonsembedded in said press seals.

5. The lamp of claim 1 wherein the ratio of the TH of said terminalportions to the TPI of said central portion of said filament is greaterthan about 1.05.

2. The lamp of claim 1 wherein the length of each of said terminalportions is less than one-third the total coiled length of saidfilament.
 3. The lamp of claim 1 wherein the space between turns of saidterminal portions is sufficient to prevent any substantial shorting outof individual turns.
 4. The lamp of claim 1 wherein the ends of saidfilament are threaded into coiled lead-in wires, said lead-in wiresbeing connected to ribbons embedded in said press seals.
 5. The lamp ofclaim 1 wherein the ratio of the TPI of said terminal portions to theTPI of said central portion of said filament is greater than about 1.05.